You think you've found a 'loose' machine. Good. Take your bankroll and play it through the machine once. Do not replay any coins/tokens that you have won. After you have played your bankroll through the Slot Machine, take any winnings and quit. This method will practically guarantee that you will leave the 'session' with some money (if not an actual jackpot).
If your machine seems to be on an upward trend, you may opt to play your winnings (up to the amount of your initial bankroll) through the machine once more. Playing the same Slot Machine beyond this point, however, is risky.

The step down method

For the sake of demonstration, let's say you've chosen to start playing a $5 slot machine. Play the machine until you've played one-half of your initial bankroll. Stop. Count the money/tokens in the coin tray. Add it to what's left of your initial bankroll. If the total amount exceeds your original session stake (bankroll), play the remaining amount of your initial bankroll through the same machine. If it does not total your initial stake, 'step down' to the next denomination machine (in this case, a $1 machine). At this machine, play one-half of what's left of your starting bankroll. Stop. As before, count the money/tokens you have won and add it to what remains of your original stake. If the combined total exceeds your starting bankroll, 'step up' to a $5 slot machine once again. If your combined winnings do not equal or exceed your original bankroll, 'step down' to the next denomination machine (a quarter machine) ... and repeat the process. If you don't actually come away from the casino a winner, you will have stretched your playing time tremendously without exceeding your original bankroll.

The percentage play method

This particular strategy involves playing a fixed percentage of your bankroll for each 'pull'. For example, let's say your session stake is $100. You have chosen to play a $1 slot machine. You will play a maximum of 5% of your stake each pull. Therefore, you would play up to 5 coins on the first pull. If you win, you may continue to play at 5%. If not, then you limit your next pull to 5% of what remains of your bankroll. And so on.

The Blitz method

This strategy is not for the weak-of-heart or for those whose bankrolls are not sufficiently healthy. You must also be relatively certain that you have chosen a 'loose' machine. Play your entire bankroll through the machine regardless of whether you win or lose. Why? Your objective is to "force" the machine to pay you a very nice mid-level jackpot. If you are lucky (and you have indeed picked a 'loose' machine), you may receive more than one good-sized pay-out.
The downside is: if your chosen slot machine does not respond the way you thought it would, your bankroll is gone.